2015 Silverado P0171 and P0174 Lean Both Banks - Rough Idle

Question

My truck is throwing both lean codes on Bank 1 and Bank 2. It idles very poorly at stoplights and sometimes feels like it is going to stall, but it seems to accelerate fine on the highway. I can hear a slight whistling sound from the engine bay when it is idling in the driveway.

Luis Hernandez · Accepted Answer

Found the issue was a ruptured PCV diaphragm built into the driver side valve cover. I performed a smoke test and saw smoke pouring out of the tiny vent hole on the cover. Since the PCV is integrated, I had to replace the entire valve cover assembly (GM Part #12642655). Total cost was about $110 for the part and an hour of my time; it fixed the lean codes and the rough idle immediately.

dieselmike88 · Answer

Check your Long Term Fuel Trims at idle versus 2500 RPM. If the trims improve at higher RPMs, you definitely have a vacuum leak behind the throttle body.

Rachel Nguyen · Answer

On these L83 engines, the brake booster vacuum line is a common failure point. Inspect the plastic clips where it connects to the intake manifold for cracks.

hd_trucker_77 · Answer

The whistling sound is a dead giveaway for a vacuum leak. Spray some intake cleaner around the intake manifold gaskets while the engine is running and listen for a change in RPM.

2015 Silverado P0171 and P0174 Lean Both Banks - Rough Idle

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